Turbochargers are a type of forced induction system. They compress the air flowing into an engine, thus boosting the engine's horsepower without significantly increasing weight. Turbochargers use the exhaust flow from the engine to spin a turbine, which in turn drives an air compressor. Since the turbine spins about 30 times faster than most car engines and it is hooked up to the exhaust, the temperature in the turbine is very high. Additionally, due to the resulting high velocity of flow, turbochargers are subjected to noise and vibration. Such conditions can have a detrimental effect on the components of the turbocharger, particularly on the rotating parts such as the turbine rotor, which can lead to failure of the system. Additionally, thermal growth or expansion due to the temperature changes must be designed for which can lead to inefficencies as a result of unwanted gaps under certain conditions.
Turbochargers are in use in connection with laree diesel engines as well as with smaller, passenger car power plants. The design and function of turbochargers is described in detail in the prior art, for example, U.S. Pat. Nos. 4,705,463, 5,399,064, and 6,164,931, the disclosures of which are incorporated herein by reference.
Turbocharger units typically include a turbine operatively connected to the engine exhaust manifold, a compressor operatively connected to the engine air intake manifold, and a shaft connecting the turbine wheel and compressor wheel so that the rapidly rotating turbine wheel drives the compressor wheel. The shaft extends through a bearing housing and is mounted for rotation in bearings. The bearings are most often free-floating bearings. Crankcase lubricant under pressure is pumped through the free floating bearings to lubricate the rotating bearing interfaces, as well as the thrust surfaces that limit axial excursions of the shaft.
In addition to performing the useful work as described above, turbochargers must be designed to combat two significant problems: first, oil should not be allowed to escape from the bearing housing into the turbine or compressor housing, and from there into the environment, and second, the high temperature of the turbine must not be allowed to adversely affect the lubricating oil in the bearing housing.
More specifically, turbocharged vehicles are required to meet increasingly stringent emissions standards. It is a challenge to contain lubricant within the bearing housing, considering that lubricating oil is pumped in under pressure, at a high flow rate, to lubricate and remove heat from a turbine shaft which extends through the turbine housing and rotates at up to 350,000 rpm. Although barriers are set up in the turbocharger, some amount of the lubricant will escape from the bearing housing into either the turbine housing or the compressor housing. This lubricant is ultimately emitted into the environment via the exhaust, contributing to emissions.
Regarding the second mentioned problem, temperatures of about 740° C. occur in the exhaust gas turbine in the case of diesel engines and about 1,000° C. in the case of Otto-cycle engines. The transfer of high temperatures from the turbine portion of the turbocharger to the bearing housing can lead to oxidation of the lubricating oil within the bearings and on the walls of the center housing.
It is known to use heat shields in order to protect the bearing housing from the high temperatures of the exhaust gas turbine. Heat shields are described for example in U.S. Pat. Nos. 4,613,288; 4,969,805; 5,026,260; 5,214,920; 5,231,831: and 5,403,150. According to conventional wisdom, the heat shield is a piece of metal in the shape of a flat disc interposed between the turbine and bearing housing and able to withstand exposure to high temperatures. Where variable geometry guide vanes are used by the turbocharger, the positioning of the heat shield must be such so as not to interfere with movement of the vanes or the turbine wheel, even with thermal growth of the components of the turbocharger.
In U.S. Pat. No. 7,097,432 to Lombard, a VTG turbocharger is shown having a heat shield mounted between a turbine housing and a center housing. The Lombard device is shown in FIGS. 1 and 2 and has a turbine housing 12, a center housing 14 a compressor lousing 16, a turbine wheel 18, housing bolts 40, rotatable guide vanes 90 and a heat shield 92. The heat shield 92 is mounted concentrically with the turbine wheel 18 by being clamped or sandwiched along a periphery thereof by the turbine and center housings 12 and 14. The housing bolts 40 apply compression to maintain the heat shield 92 in place during operation of the turbocharger. The Lombard assembly, while typical of heat shield assemblies for turbochargers, can suffer from the drawback of movement of the heat shield 92 with thermal growth of the turbine housing 12. Additionally, to account for thermal growth and assembly tolerances when positioning the heat shield 92 in between the housings 12 and 14 so that the turbine wheel 18 and guide vanes 90 have adequate clearance, larger gaps are provided which can decrease the efficiency of the turbocharger.
Additionally, where a turbocharger assembly seeks to integrate the housings to eliminate the clamping joint, other connection means becomes necessary. Additional components, such as connection structures. e.g., bolts, are costly and can be subject to failure over time due to the extreme conditions that the connection structures are subjected to.
Thus, there is a need for a heat shield assembly for improved connection with the turbine and/or bearing housing. There is a further need for such an assembly that accounts for thermal growth of the housing and/or vane ring assembly while maintaining efficiencies. There is a yet a further need for such a system and method that is cost effective and dependable. There is additionally a need for such a system and method that facilitates manufacture, assembly and/or disassembly.